1. Field of the Invention
The present invention relates to a magnetic motion sensor used as a rotation sensor in combination with a rotor and, more particularly, to a magnetic motion sensor with enhanced reliability.
2. Description of the Related Art
Heretofore, a noncontact rotation sensor to detect rotation of a rotor is known in which a change in magnetic field generated when a magnetic material rotates together with the rotor is detected by a magnetic sensor to output a pulse signal relating to the rotation.
Rotation sensors as shown in FIGS. 19 and 20 are used to detect a rotation of tire in ABS (=antilock brake system), which is a brake control system for automobiles.
The rotation sensor as shown in FIG. 19 is constructed such that a gear (rotor) 201 made of a magnetic material is attached to a rotating shaft (no shown) of tires, and a Hall IC 203 with a Hall element 202 mounted thereon is disposed radially outside of the rotor 201 wherein the Hall IC 203 is disposed such that the single-sided Hall element 202 is opposed to each tooth of the rotor 201 (rotor tooth 204), and a magnet 205 (hereinafter referred to as “back magnet”) is disposed on the opposite side of the Hall IC 203.
A magnetic flux formed in a direction from the back magnet 205 to the rotor 201 passes densely through the Hall element 202 when the Hall IC is right in front of a rotor tooth 204. It disperses when the Hall IC 203 is faced to a valley between rotor teeth. Thereby, the Hall element 202 can detect strong and weak changes in a magnetic field. The Hall IC 203 converts the changes in the magnetic field (output signals of the Hall element) into pulses being binary signals in a circuit inside the Hall IC to output the signals. When a passage speed of rotor teeth is detected from widths or cycles of these pulses, a turning angle velocity or number of rotations of the rotating shaft can be determined.
The rotation sensor as shown in FIG. 20 is constructed such that a rotation disk (rotor) 212 with plural magnets 211 each of which has a polarity alternated is attached to a rotating shaft of tires, and a Hall IC 214 with a Hall element 213 is opposed to the rotor 212. The Hall IC 214 is disposed such that the single-sided Hall element 213 is opposed to magnets (hereinafter conveniently referred to as “rotor teeth”) 211. When the Hall IC 214 is right in front of the rotor teeth, a magnetic flux from the rotor teeth passes through the Hall element 213, while when the Hall IC 214 is right in front of an area between teeth, the magnetic flux from the rotor teeth 211 does not pass through the Hall element 213, so that magnetic field change signals are obtained from the Hall element as in the case of the rotation sensor of FIG. 19, whereby outputs of pulses can be taken out from the Hall IC.
According to the art disclosed in Japanese Patent. Application Laid-Open No. 7-209311, three Hall elements facing to a rotor are aligned, magnetic field change wave forms wherein phases thereof deviate from the respective Hall elements are detected, and binary signals obtained from the respective magnetic field change wave forms are synthesized, whereby pulses each having a short period of time are obtained even in case of a low speed rotation.
Incidentally, in case of manufacturing the rotation sensor of FIG. 19, the Hall IC is provided as a part prepared by incorporating the Hall element, the back magnet, and a circuit with each other, while the rotor is provided as an assembly of a bearing called usually a hub, and a combination of the Hall IC with the rotor is completed in an assembly line of the hub for the first time. In case of the rotation sensor of FIG. 20 also, the Hall IC and the rotor are provided as separate parts, and a combination of them is completed in an assembly line of a hub for the first time. Moreover, the hub is assembled with respect to a rotation shaft of tires in an automobile assembly line.
In the present specification, a part of the Hall IC among these rotation sensors is treated as a magnetic motion sensor for detecting changes in a magnetic field appearing as a result of spatial movement.
The rotation sensors of the prior art involves the following problems.
(1) In rotor teeth, there are dispersions in dimensions (height, circumferential width, thickness, pitch) dependent upon its forming accuracy. For this reason, outputs of pulses from a Hall IC involve dispersions in a pulse width and a pulse cycle even when a rotor rotates at a constant speed. Hence, a passage speed of an individual rotor tooth cannot be detected with a high degree of accuracy.
(2) Irrespective of a rotating direction of a rotor, the Hall IC outputs pulses in common with both the directions, and thus, its real rotating direction cannot be detected.
(3) Although the Hall IC detects a magnitude of a magnetic field, such magnitude of a magnetic filed depends on a size of a gap between a rotor tooth and the Hall IC. Accordingly, there is a case where changes in a magnetic field cannot be detected dependent on a size of the gap by means of rotor teeth. In this respect, however, there is no effective means for judging the fact that the gap has a suitable size at the time of installing the Hall IC.
(4) In the manner of FIG. 19, there are differences among intensities in a magnetic field detected by the Hall IC dependent upon differences in installing positions or characteristics of a magnet. In this respect, however, there is no means for determining differences in intensities of the magnetic field at the time of installing the magnet.
(5) As to a pulse width of pulses output from the Hall IC, binarization is made by applying a threshold value to an intensity of a magnetic field detected by the Hall IC, so that it depends on a size of a gap between a rotor tooth and the Hall IC. In other words, even if a rotational velocity exhibits the same value, there is a case where a pulse width becomes narrow or wide dependent upon an installed position of the Hall IC. It makes a processing for rotating velocity and accelerated velocity in a device in the subsequent stage by which outputs from the Hall IC are received difficult.
(6) Concerning not only automobiles, but also objects to which a detection of rotation is applied, there is a case where noises due to electromagnetic factors (ignition, motor driving and the like) or mechanical factors (blurring in a gap) arise. A detection of rotation or a communication with superior machinery is adversely affected by these noises.
(7) When output signals from the Hall element are simply binarized, pulse strings similar to a concavo-convex profile of the rotor teeth are obtained. However, a pulse width or a pulse interval becomes broadened in case of a comparatively low-speed rotation, so that a waiting time for detecting rotations in the subsequent device is lengthened. Moreover, in either a case of a remarkably low-speed rotation or a case of stopping rotation, no pulse is obtained, and in such case, it cannot be discriminated even whether the Hall element is active or inactive in the subsequent device.
In addition, a part of the Hall IC (magnetic motion sensor) is provided as a separate part from a rotor as mentioned above, so that it is difficult to decide that where is a cause for the above enumerated problems and failures accompanied therewith in an assembled rotation sensor. As a result, a reliability of a rotation sensor cannot be assured.